Nuclear Magnetic Resonance (NMR)

MagneticNuclear Resonance

In the Nucleus

Involves Magnets

In the Nucleus

Introduction

• NMR is the most powerful tool available for

organic structure determination.

• It is used to study a wide variety of nuclei:

– 1H

– 13C

– 15N

– 19F

– 31P

3

Nuclear Spin

• A nucleus with an odd atomic number or an

odd mass number has a nuclear spin.

• The spinning charged nucleus generates a

magnetic field.

External Magnetic Field

When placed in an external field, spinning protons act like bar magnets.

Two Energy States

The magnetic fields of the spinning nuclei will align either withthe external field, or against the field.

A photon with the right amount of energy can be absorbed and cause the spinning proton to flip.

• The nucleus of a hydrogen atom has a very weak magnetic spin, it behaves like a weak compass needle.

• If a molecule containing hydrogen is placed in a strong magnetic field, the magnetic hydrogen nucleus can line up with the field or line up against it!

N SS N

N SN S

• Which is the high energy orientation?

Nucleus spin aligned with the field – Low energy!

Nucleus spin aligned against the field –High energy!

N SN S

Add Energy

N SS N

Aligned = Low Energy

Excited state = High energy

N SS N

Energy Released

Back to low energy ground state

• When the spin falls back into line with the magnetic field it releases energy. We detect this energy and it provides information on:

• The environment of the hydrogen in the molecule

• How many hydrogen atoms are in that environment.

NMR is a very detailed method of chemical analysis for ORGANIC compounds. It can tell us the number of hydrogen atoms in a molecule and their related positions in the carbon chain.

The nucleus of each hydrogen atom behaves like a tiny magnet, which usually lines up with an applied magnetic field. However, if we add energy, the tiny magnet can flip over so that it aligns against the magnetic field.

When the external energy is removed, the magnetic nucleus must, once again, fall back in line with the magnetic field and release its extra energy. We detect this released energy and use it to gather information about the hydrogen which was excited.

The NMR Spectrometer

OBTAINING SPECTRA

• a liquid sample is placed in a tube which spins in a magnetic field

• solids are dissolved in deuterated solvents (CDCl3) or solvents without H‟s

(CCl4 ) [solvents with hydrogen atoms in them will produce peaks in the

spectrum]

• TMS, tetramethylsilane, (CH3)4Si, is added to provide a reference signal.

•In practice, the radio wave interacts with the nuclei to cause a rotation

of the nuclear magnets.

•The rotating magnet produces an electrical current in a wire placed

around the sample, and this is what is detected.

• In principle, a detector could record the difference in the signal that it

receives compared to the original signal and cause an absorption peak

to appear on the chart recorder.

• non-toxic liquid - SAFE TO USE

• inert - DOESN‟T REACT WITH COMPOUND BEING ANALYSED

• has a low boiling point - CAN BE DISTILLED OFF AND USED AGAIN

• all the hydrogen atoms are chemically equivalent - PRODUCES A

SINGLE PEAK

• given the chemical shift of d = 0

• the position of all other signals is measured relative to TMS

TETRAMETHYLSILANE - TMS

The molecule contains four methyl groups attached

to a silicon atom in a tetrahedral arrangement. All

the hydrogen atoms are chemically equivalent.

PROVIDES THE REFERENCE SIGNAL

• If all the hydrogen atoms in a compound are bonded to a common carbon atom, then only one absorption would be observed in the 1H NMR spectrum of the molecule.

•For example, the methane molecule, CH4, has four chemically equivalent hydrogen atoms and has only one peak in its NMR spectrum.

• Hydrogen in different structural arrangements (or chemical environments) give rise to peaks at different positions in the NMR spectrum.

For example, the butane molecule, CH3−CH2−CH2−CH3, gives rise to two peaks in the NMR spectrum.

One peak is due to resonance involving the terminal hydrogens in the methyl groups, −CH3, at the end of the molecule; the other is due to the hydrogens in the methylene groups, −CH2−, in the centre of the molecule.

This occurs because the nuclei of the hydrogen atoms are shielded by other electrons in the molecule to different extents.

The hydrogen nuclei are said to be in different chemical environments.

chemical environments of Hydrogen

• each proton type is said to be chemically shifted relative to a standard

(usually TMS)

• the chemical shift is the difference between the field strength at which it

absorbs and the field strength at which TMS protons absorb

• the delta (d) scale is widely used as a means of reporting chemical shifts

• the TMS peak is assigned a value of ZERO (d = 0.00)

• all peaks of a sample under study are related to it and reported in

parts per million

CHEMICAL SHIFT

Information from 1H-nmr spectra:

1. Number of signals: How many different types of hydrogen's in the molecule.

2. Position of signals (chemical shift): What types of hydrogen's.

3. Relative areas under signals (integration): How many hydrogen's of each type.

4. Splitting pattern: How many neighboring hydrogen's.

LOW RESOLUTION SPECTRA

LOW RESOLUTION SPECTRUM OF 1-BROMOPROPANE

• low resolution nmr gives 1 peak for each environmentally different group of protons

HIGH RESOLUTION SPECTRA

HIGH RESOLUTION SPECTRUM OF 1-BROMOPROPANE

The broad

peaks are split

into sharper

signals

The splitting pattern depends on the number of hydrogen atoms on adjacent atoms

• high resolution gives more complex signals - doublets, triplets, quartets, multiplets

• the signal produced indicates the number of protons on adjacent carbon atoms

Four

Two

1. Number of signals: How many different types of hydrogens in the molecule.

One Two Two

One One

Two

2. Position of signals (chemical shift): what types of hydrogens.

3. Integration (relative areas under each signal): how many hydrogens of each type.

Number of peaks = number of chemically different H‟s on adjacent atoms + 1

1 neighbouring H 2 peaks “doublet” 1:1

2 neighbouring H‟s 3 peaks “triplet” 1:2:1

3 neighbouring H‟s 4 peaks “quartet” 1:3:3:1

4 neighbouring H‟s 5 peaks “quintet” 1:4:6:4:1

Signals for the H in an O-H bond are unaffected by hydrogens on adjacent

atoms - get a singlet

MULTIPLICITY (Spin-spin splitting)• low resolution nmr gives 1 peak for each environmentally different group of protons

• high resolution gives more complex signals - doublets, triplets, quartets, multiplets

• the signal produced indicates the number of protons on adjacent carbon atoms

Spin–spin splitting is illustrated in the high-resolution 1H NMR spectrum of ethanol

4. Splitting pattern: How many neighboring hydrogens

cyclohexane

a singlet 12H

2,3-dimethyl-2-butene

C

CH3

C

H3C

H3C

CH3

a singlet 12H

benzene

a singlet 6H

ethyl bromide

a bCH3CH2-Br

a triplet 3Hb quartet 2H

1-bromopropane

a b c

CH3CH2CH2-Br

a triplet 3Hb hextate 2Hc triplet 3H

Chloropropane

a b aCH3CHCH3

Cl

a doublet 6Hb septet 1H

ethanol

a c b

CH3CH2-OH

a triplet 3Hb singlet 1Hc quartet 2H

Propanol

a b d cCH3CH2CH2-OH

a triplet 3Hb complex 2Hc singlet 1Hd triplet 2H

Integration trace

• In addition to the NMR spectrum, the NMR spectrometer has drawn

what is termed „an integrated spectrum trace‟.

• The height of each step is a measure of the area under the peak.

•It is proportional to the number of hydrogen atoms (protons)

resonating at this point in the NMR spectrum.

• In this case there are three steps, which are in the ratio of 1 : 2 : 3

(from left to right).

MRI

• Magnetic resonance imaging, noninvasive

• “Nuclear” is omitted because of public’s fear

that it would be radioactive.

• Only protons in one plane can be in resonance

at one time.

• Computer puts together “slices” to get 3D.

• Tumors readily detected.

Radio waves then cause thehydrogen atoms in the watermolecules of the body toresonate.

Each type of body tissue emitsa different signal, reflectingthe different hydrogen densityof the tissue.

Computer software thentranslates these signals into athree-dimensional picture.

Magnetic resonance imaging (MRI) uses NMR formedical diagnosis.

The patient is placed inside a cylinder that contains avery strong magnetic field (usually generated by asuperconducting magnet).

MRI does not ‘see’ bone and can only produce images of soft tissues such as blood vessels, cerebrospinal fluid, bone marrow and muscles.

This occurs because the amount of water in bone is very small compared to the amount in soft tissue.

MRI is used to detect brain tumors, damage caused by multiple sclerosis (MS) or strokes, joint injuries, heart disease (caused by the narrowing of arteries) and herniated discs.

It is regarded as a harmless procedure except to those patients who have metal implants, such as a pacemaker, joint pins, shrapnel or artificial heart valves.